1. Field of the Invention
The present invention relates to a packaging material suitable for a packaging bag or a lid material of a packaging container suitable for heat-treating a processed food and various food materials in a microwave oven or the like, and heat-sterilizing a medical instrument.
2. Description of the Related Art
Various food materials to be heat-treated by a microwave oven in a packaged state are sold. In this case, a food material is often sold as a sealed packaged body sealed and packaged by a packaging material. When such a sealed and package food material is heated in the microwave oven, moisture and the like on the inside of the food material evaporates, the internal pressure of the sealed packaged body rises, the packaging material swells, and, when the food material is further heated, the packaging material bursts.
Therefore, a hole is opened in a part of the packaging material or the packaging material is opened in advance before the heating to prevent the packaging material from bursting. However, it is troublesome to open the hole in the packaging material or open the packaging material before the heating because a tool, such as scissors, is necessary.
There is also a problem in that a user sometimes forgets to open a hole and heats the packaging material.
To solve these problems, there has been proposed a packaging material including a function of automatically forming an opening for discharging water vapor making use of a rise in the internal pressure of a sealed packaged body.
For example, Japanese Patent Publication No. H8-91450 describes a degassing structure for a plastic bag or container in which a first film and a second film, on the entire surface of which micropores are opened, are discontinuously bonded by an adhesive excluding one side end portion, and the outer side and the inner side of a bag are caused to communicate via the micropores and the one side end portion, whereby a passage of gas is provided to exhaust generated steam.
In such a degassing structure, since the bag inside and the outside air communicate, the degassing structure is not considered sealed packaging and it is likely that dust and bacteria intrude into a packaging bag or a packaging container. Since the micropores are opened over the entire surface of the second film, a packaged liquefied food passes through the micropores and comes into direct contact with the adhesive that bonds the second film and the first film. Therefore, there is a problem in that a part of the adhesive melts and mixes into the food.
A packaging film that solves these problems is disclosed in Japanese Patent Publication No. 4817583.
FIG. 14A is a diagram showing a packaging bag 1 formed of the packaging film described in Japanese Patent Publication No. 4817583. FIG. 14B is a diagram of a II-II section of FIG. 14A seen from an arrow direction.
The packaging film is a film obtained by sticking together a first film 2 formed of a stretched film and a second film 3 formed of a non-oriented film. A low-melting point heat seal agent (hereinafter simply referred to as “heat seal agent”) 4 is applied to a part of a region of a sticking portion. A cut line 5 passing a place of the first film 2 where a heat seal agent 4 is applied is engraved in the first film 2 of the packaging film. Note that, in FIG. 14A, a longitudinal-direction bonded section 6, lateral-direction bonded sections 7 and 9, and a bag seal end portion b are shown.
When the film is heated in a microwave oven, the heat seal agent 4 is melted by the heat of steam generated from a content 8. Laminate strength between the first film 2 and the second film 3 markedly decreases and the second film 3 softens with the heat. Since the cut line 5 is engraved in the first film 2, when the internal pressure of the bag rises, the second film 3, which is the non-oriented film, in the place where the heat seal agent 4 is applied easily stretches in a direction perpendicular to the direction of the cut line 5. On the other hand, in a portion where the heat seal agent 4 is not applied, the softening of the second film 3 occurs in the same manner. However, since a bonded state is firmly kept with the first film 2 that is hardly softened even if heated by water vapor, the second film 3 cannot easily stretch and expand even though the cut line 5 is engraved in the first film 2. Therefore, a small hole is formed in a crossing point “a” of a boundary portion between the place where the heat seal agent 4 is applied and the place where the heat seal agent 4 is not applied and the cut line 5.
As explained above, in the packaging film described in Japanese Patent Publication No. 4817583, since the second film 3 (the non-oriented film) itself is not particularly processed, a perfect sealed state is kept. Further, the adhesive that bonds the first film 2 (the stretched film) and the second film 3 (the non-oriented film) does not come into contact with the content.
Incidentally, the non-oriented film stretches differently depending on the type of a material. In particular, depending on the type of the non-oriented resin film, some non-oriented films stretch with a weak force when heated and other non-oriented films stretch in a manner called necking in which stretching occurs in one part at a time.
A microwave oven to be used has fluctuations in power depending on the type thereof. The non-oriented film is slowly heated when warmed by a microwave oven having weak power. The pressure in the bag slowly rises. If the internal pressure slowly rises, the non-oriented film is slowly stretched.
In the method described in Japanese Patent Publication No. 4817583 for simply opening a hole in the film in a boundary line between the applied portion of the heat seal agent and the firmly laminated portion, a steam hole sometimes cannot be drilled stably depending on the type of film or because a heating condition changes.
When the packaging film is actually used in the market, since several tens of thousands of foods are sometimes consumed in one day, steam holes need to be stably opened in all the packaging films. However, in particular, in heating by a microwave oven in a home, there are various conditions for consumers to individually perform the heating. Differences in the conditions are factors of instability in opening holes. In the packaging material including the conventional hole opening mechanism described above, the steam hole is sometimes not opened under special conditions and materials to be used are sometimes limited.